1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor devise with an electric converter element such as thermoelectric or electrothermal converter, which is applicable to various sensors, generators, and actuators using heat, such as an Infrared (IR)-ray sensor, flow sensor, gas sensor, pressure sensor, vacuum sensor, IR-ray generator, and manipulator.
2. Description of the Prior Art
A semiconductor sensor device or semiconductor micro-sensor using heat is typically comprised of a semiconductor substrate, a heat-sensing or heat-input microstructure formed over the substrate and thermally shielded or separated therefrom, and an electronic circuit for processing an electric output signal from the microstructure. The microstructure usually has a thermoelectric converter element to produce the electric output signal according to the heat or temperature of the microstructure.
An example of the conventional semiconductor sensor devices of this sort is shown in FIGS. 1 and 2, which serves as an IR-ray sensor This sensor device is disclosed in the Japanese Non-examined Patent Publication No. 8-105794 published in 1996.
As shown in FIGS. 1 and 2, this conventional semiconductor sensor device includes a lot of rectangular diaphragms 613 as the heat-input microstructures, which are arranged in a matrix array on a semiconductor substrate 601.
As shown in FIG. 2, although roughly illustrated, a scanning circuit 602 is formed on a main surface of the semiconductor substrate 601. The scanning circuit 602 includes Metal-Oxide-semiconductor Field-Effect Transistors (MOSFETs) (not shown). Polysilicon vertical selection lines 603 are formed over the scanning circuit 602 to scan or select the diaphragms 613.
A silicon dioxide (SiO.sub.2) layer 635 is formed to cover the scanning circuit 602 and the vertical selection lines 603. Cavities 604 with a same rectangular plan shape are formed in the SiO.sub.2 film 605.
Aluminum (Al) ground lines 606 and aluminum signal lines 607 are formed on the SiO.sub.2 layer 605. Titanium (Ti) bolometers 608 serving as thermoelectric converter elements are formed on the SiO.sub.2 layer 605 to be overlapped with the corresponding cavities 604. The signal lines 607 are electrically connected to the scanning circuit 602 through contact holes 612 penetrating the SiO.sub.2 layer 605.
Another SiO.sub.2 layer 609 is formed to cover the bolometers 608, the ground lines 606, the signal lines 607, and the exposed SiO.sub.2 layer 605.
An IR-ray absorption layer 610 is selectively formed on the SiO.sub.2 layer 609 to be overlapped with the diaphragms 613. The layer 610 is made of titanium nitride (TiN).
As shown in FIGS. 1 and 2, folded slits 611a and 611b are formed to penetrate the SiO.sub.2 layers 609 and 605 and to surround the corresponding zigzag-shaped bolometers 608. The slits 611a and 611b extend to the underlying cavities 604 in the SiO.sub.2 layer 605, thereby defining the rectangular diaphragms 613 which are matrix-arranged over the substrate 601. The diaphragms 613 thus defined by the patterned SiO.sub.2 layer 609 are thermally separated from the substrate 601 by the corresponding cavities 604 and from the adjoining parts of the SiO.sub.2 layers 609 and 605 by the slits 611a and 611b. Thus, it is said that the diaphragms 613 are thermally shielded or isolated from the substrate 601. The bolometers 608 are located on the corresponding diaphragms 613.
As seen from FIGS. 1 and 2, each of the diaphragms 613 has two legs 613a and 613b that are mechanically connected to the substrate 601 through the remaining SiO.sub.2 layer 605. Each of the legs 613a and 613b is sandwiched by the adjoining slits 611a and 611b.
Each of the bolometers 608 is comprised of a zigzag-shaped central part 608c and two end parts 608a and 608b located on the legs 613a and 613b of a corresponding one of the diaphragms 613. The end parts 608a and 608b of the bolometer 608 are located on the legs 613a and 613b of the diaphragm 613 to extend along them, respectively. The end parts 608a and 608b of the bolometer 608 are electrically connected to the signal lines 607 which are electrically connected to the scanning circuit 602.
The cavities 604 formed in the SiO.sub.2 layer 605 are implemented by forming a sacrificial polysilicon layer, patterning the sacrificial polysilicon layer, and removing the patterned, sacrificial polysilicon layer. This removing process is performed by wet etching while an etching solution is contacted with the sacrificial polysilicon layer through the slits 611a and 611b.
With the conventional semiconductor sensor device shown in FIGS. 1 and 2, all the rectangular diaphragms 613 arranged on the substrate 601 in a matrix array are electrically scanned by the scanning circuit 602 on operation.
When an incident IR-ray is irradiated to the diaphragms 613, it is absorbed by the IR absorption layer 610 to thereby change the temperature of the diaphragms 613. The temperature change thus caused is converted to an electric output signal by the bolometers 608 on the diaphragms 613 and then, the electric output signal is read out to the outside of the conventional semiconductor sensor device.
The above-described conventional semiconductor sensor device shown in FIGS. 1 and 2 has the following problems.
A first problem is that the thermal shielding or blocking capability of the diaphragms 613 is unsatisfactory. This problem is applicable to any other semiconductor sensor devices.
Each of the diaphragms 613 is mechanically connected to the substrate 601 by the elongated legs 613a and 613b. The end parts 608a and 608b of the corresponding bolometer 608 a relocated on the legs 613a and 613b to thereby electrically connect the bolometer 608 to the signal lines 607. The end parts 608a and 608b are typically made of popular metal such as titanium (Ti) to decrease their electric resistance. Since metals with a high electrical conductivity generally have a high thermal conductivity, the heat generated in each diaphragm 613 tends to be readily transmitted to the substrate 601. This means that the thermal shielding or blocking capability of each diaphragm 613 will degrade.
The bolometer 608 may be made of oxide semiconductor while the end parts 608a and 608b thereof are made of metal. However, in this case, there is the same problem as above.
This first problem can be solved by decreasing the cross section of the legs 613a and 613b of the diaphragm 613. However, in this case, there arises another problem that the mechanical strength of the legs 613a and 613b is lowered The decrease of the mechanical strength of the legs 613a and 613b increases the danger that the diaphragm 613 is mechanically contacted with the underlying SiO.sub.2 layer 605 due to fluctuation or deviation of the process parameters in the fabrication process sequence of the conventional semiconductor sensor device, resulting in lowering of the fabrication yield.
A second problem is that the sensitivity of the bolometers 608 is unsatisfactorily low. This is because the bolometers 608 are made of Ti having a Temperature Coefficient of electric Resistance (TCR) as low as approximately 0.25% /K.
The bolometer 608 may be made of a vanadium oxide (VO.sub.x) or titanium oxide (TiO.sub.x). In this case, however, vanadium is not used in the popular fabrication processes of silicon ICs and as a result, it requires a dedicated process line. This means that the vanadium-based bolometer is difficult to be actually utilized.
If TiO.sub.x is used for the bolometer 608, there arises another problem that the 1/f noise of the bolometer 608 becomes high due to the high electrical resistivity of TiO.sub.x.
Additionally. U.S. Pat. No. 5,2136,976 issued in 1994 discloses that the bolometer is made of vanadium oxide (V.sub.2 O.sub.3 or VO.sub.x) or titanium oxide (TiO.sub.x).
The Japanese Non-examined Patent Publication No. 6-147993 published in 1994 discloses that the bolometer is made of polysilicon.
The Japanese Non-examined Patent Publication No. 5-40064 published in 1993 discloses that the thermoelectric converter element (i.e., bolometer) is implemented by using the temperature dependence of the backward saturation current of a Schottky diode.